Methods for doping fin field-effect transistors and fin field-effect transistor

ABSTRACT

A method of doping a fin field-effect transistor includes forming a plurality of semiconductor fins on a substrate wherein each semiconductor fin of the plurality of semiconductor fins has a top surface and sidewalls. The method includes forming a gate stack over the top surface and sidewalls of each semiconductor fin. The method includes removing a portion of a first semiconductor fin exposed by the gate stack. The method includes growing a first stressor region connected to a remaining portion of the first semiconductor fin. The method includes exposing a second semiconductor fin to a deposition process to form a dopant-rich layer comprising an n-type or a p-type dopant on the top surface and the sidewalls of the second semiconductor fin. The method includes diffusing the dopant from the dopant-rich layer into the second semiconductor fin using an annealing process.

PRIORITY CLAIM

The present application is a continuation of U.S. application Ser. No. 14/621,035, filed Feb. 12, 2015, which is a continuation of U.S. application Ser. No. 12/768,884, filed Apr. 28, 2010, now U.S. Pat. No. 8,980,719, issued Mar. 17, 2015, which are incorporated herein by reference in their entireties.

RELATED APPLICATIONS

The present application is related to U.S. patent application Ser. No. 12/707,788, filed on Feb. 18, 2010, (Docket Nos. TSMC2008-0432, T5057-R007U), titled MEMORY POWER GATING CIRCUIT AND METHODS; Ser. No. 12/758,426, filed on Apr. 12, 2010, (Docket Nos. TSMC2008-0582, T5057-Y048U), titled FINFETS AND METHODS FOR FORMING THE SAME; Ser. No. 12/731,325, filed on Mar. 25, 2010, (Docket Nos. TSMC2008-0597, T5057-B033U), titled ELECTRICAL FUSE AND RELATED APPLICATIONS; Ser. No. 12/724,556, filed on Mar. 16, 2010, (Docket Nos. TSMC2008-0598, T5057-K002U), titled ELECTRICAL ANTI-FUSE AND RELATED APPLICATIONS; Ser. No. 12/757,203, filed on Apr. 9, 2010, (Docket Nos. TSMC2009-0148, T5057-Y085U), titled STI STRUCTURE AND METHOD OF FORMING BOTTOM VOID IN SAME; Ser. No. 12/797,839, filed on Jun. 10, 2010, (Docket Nos. TSMC2009-0278, T5057-K099U), titled FIN STRUCTURE FOR HIGH MOBILITY MULTIPLE-GATE TRANSISTOR; Ser. No. 12/831,842, filed on Jul. 7, 2010, (Docket Nos. TSMC2009-0343, T5057-Y093U), titled METHOD FOR FORMING HIGH GERMANIUM CONCENTRATION SiGe STRESSOR; Ser. No. 12/761,686, filed on Apr. 16, 2010, (Docket Nos. TSMC2009-0442, T5057-Y125U), titled FINFETS AND METHODS FOR FORMING THE SAME; Ser. No. 12/766,233, filed on Apr. 23, 2010, (Docket Nos. TSMC2009-0444, T5057-K123U), titled FIN FIELD EFFECT TRANSISTOR; Ser. No. 12/757,271, filed on Apr. 9, 2010, (Docket Nos. TSMC2009-0445, T5057-Y113U), titled ACCUMULATION TYPE FINFET, CIRCUITS AND FABRICATION METHOD THEREOF; Ser. No. 12/694,846, filed on Jan. 27, 2010, (Docket Nos. TSMC2009-0646, T5057-Y165), titled INTEGRATED CIRCUITS AND METHODS FOR FORMING THE SAME; Ser. No. 12/638,958, filed on Dec. 14, 2009, (Docket Nos. TSMC2009-0738, T5057-B166), titled METHOD OF CONTROLLING GATE THICKNESS IN FORMING FINFET DEVICES; Ser. No. 12/768,884, filed on Apr. 28, 2010, (Docket Nos. TSMC2010-0028, T5057-Y228), titled METHODS FOR DOPING FIN FIELD-EFFECT TRANSISTORS; Ser. No. 12/731,411, filed on Mar. 25, 2010, (Docket Nos. TSMC2010-0057, T5057-B218), titled INTEGRATED CIRCUIT INCLUDING FINFETS AND METHODS FOR FORMING THE SAME; Ser. No. 12/775,006, filed on May 6, 2010, (Docket Nos. TSMC2010-0198, T5057-Y246), titled METHOD FOR FABRICATING A STRAINED STRUCTURE; Ser. No. 12/886,713, filed Sep. 21, 2010, (Docket Nos. TSMC2010-0646, T5057-B325), titled METHOD OF FORMING INTEGRATED CIRCUITS; Ser. No. 12/941,509, filed Nov. 8, 2010, (Docket Nos. TSMC2010-0561, T5057-B337), titled MECHANISMS FOR FORMING ULTRA SHALLOW JUNCTION; Ser. No. 12/900,626, filed Oct. 8, 2010, (Docket Nos. TSMC2010-0581, T5057-B330), titled TRANSISTOR HAVING NOTCHED FIN STRUCTURE AND METHOD OF MAKING THE SAME; Ser. No. 12/903,712, filed Oct. 13, 2010, (Docket Nos. TSMC2010-0731, T5057-R350), titled FINFET AND METHOD OF FABRICATING THE SAME; 61/412,846, filed Nov. 12, 2010, (Docket Nos. TSMC2010-0839, T5057-B388PRO), 61/394,418, filed Oct. 19, 2010, (Docket Nos. TSMC2010-0926, T5057-Y351PRO), titled METHODS OF FORMING GATE DIELECTRIC MATERIAL and 61/405,858, filed Oct. 22, 2010, (Docket Nos. TSMC2010-0928, T5057-R368PRO), titled METHODS OF FORMING SEMICONDUCTOR DEVICES.

TECHNICAL FIELD

The disclosure relates generally to integrated circuit devices, and more particularly to methods for doping fin field-effect transistors (FinFETs).

BACKGROUND

In the rapidly advancing semiconductor manufacturing industry, complementary metal oxide semiconductor (CMOS) FinFET devices may be used in many logic and other applications and are integrated into various different types of semiconductor devices. FinFET devices typically include semiconductor fins with high aspect ratios in which the channel and source/drain regions for the transistor are formed. A gate is formed over and along the sides of the portion of the fin devices utilizing the advantage of the increased surface area of the channel and source/drain regions to produce faster, more reliable and better-controlled semiconductor transistor devices. Further advantages of FinFETs include reducing the short channel effect and higher current flow.

Current FinFET technology has challenges however. For example, ion implantation is typically used to form a lightly doped drain (LDD) region. Ion implantation creates a non-conformal doping profile of the fin (for example, heavier doping at the top of the fin than the bottom of the fin, which is found closer to the substrate). This non-conformal doping profile may create issues including those associated non-uniform device turn on. By using a tilt implant, the photo resist height induced shadowing effects and pre amorphization (PAI) induced twin boundary defects will be disadvantageous.

As such, an improved fabrication method for a FinFET element is desired.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is emphasized that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.

FIGS. 1 to 8 show various stages during fabrication of a FinFET structure on a substrate according to one or more embodiments.

FIG. 9 illustrates an apparatus for performing method in accordance with an embodiment.

FIG. 10 shows schematic DC and RF bias voltages applied during the deposition of a dopant-rich layer.

FIG. 11 depicts a flowchart of a method for fabricating FinFET structures according to one embodiment.

DETAILED DESCRIPTION

The making and using of illustrative embodiments are discussed in detail below. It should be appreciated, however, that the disclosure provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative and do not limit the scope of the invention.

FIGS. 1 to 8 are perspective and cross-sectional views of FinFET structure at various stages of manufacture. FIG. 11 depicts a flowchart of a method 1100 for fabricating FinFET structures according to one embodiment of the invention.

Referring to FIG. 1 and FIG. 11, in process step 201, a substrate comprising a first fin 105 and a second fin 107 is provided. In some embodiments, substrate 101 may be a silicon substrate, a germanium substrate, or a substrate formed of other semiconductor materials. Substrate 101 may be doped with a p-type or an n-type dopant. Isolation regions such as shallow trench isolation (STI) regions 103 may be formed in or over substrate 101. The first semiconductor fin 105 and the second semiconductor fin 107 extend above the top surfaces of STI regions 103. The first semiconductor fin 105 has a top surface 111 and sidewalls 109. The second semiconductor fin 107 has a top surface 115 and sidewalls 113. Substrate 101 includes a portion in first device region 100 and a portion in second device region 200. Semiconductor fins 105 and 107 are in first device region 100 and second device region 200, respectively. In an embodiment, first device region 100 and second device region 200 are used to form one n-type FinFET and one p-type FinFET.

Referring to FIG. 2, gate dielectric layer 117 and gate electrode layer 119 are deposited in both first device region 100 and second device region 200 and over semiconductor fins 105 and 107. In an embodiment, gate dielectric layer 117 is formed of a high dielectric constant (high-k) dielectric material. The exemplary high-k materials may have k values greater than about 4.0, or even greater than about 7.0, and may include aluminum-containing dielectrics such as Al₂O₃, HfAlO, HfAlON, or AlZrO; Hf-containing materials such as HfO₂, HfSiO_(x), HfAlO_(x), HfZrSiO_(x), or HfSiON; and/or other materials such as LaAlO₃ or ZrO₂. Gate electrode layer 119 is formed on gate dielectric layer 117, and may be formed of a conductive material such as doped polysilicon, metals, or metal nitrides.

Referring to FIG. 3 and FIG. 11, in process step 203, gate electrode layer 119 and gate dielectric layer 117 are then patterned to form gate stacks. The gate stack in first device region 100 includes gate electrode 123 and gate dielectric 121. The gate stack in second device region 200 includes gate electrode 127 and gate dielectric 125. The gate stacks are over a portion of the top surfaces 111, 115 and the sidewalls 109, 113 of each of the semiconductor fins 105 and 107. In some embodiments, the exposed portions of semiconductor fins 105 and 107 may be left as they are, so that subsequent pocket and lightly doped source and drain (LDD) regions may be formed. In some alternative embodiments, the exposed portions of semiconductor fins 105 and 107 may be removed to form recesses, and semiconductor stressors may be epitaxially re-grown in the resulting recesses. In an exemplary embodiment, the semiconductor stressors in first device region 100 may comprise silicon carbon (SiC), while the semiconductor stressors in second device region 200 may comprise silicon germanium (SiGe)

Referring to FIG. 4 and FIG. 11, in process step 205, hard mask 129 is formed and patterned to cover the first device region 100, while leaving the second device region 200 uncovered. FIG. 4 is a cross-sectional view obtained from the vertical plane crossing line A-A′ in FIG. 3. Accordingly, the gate stacks are not shown.

Referring to FIG. 11, in process step 207, the process step 207 removes a native oxide layer on the exposed portion of the top surfaces 111, 115 and the sidewalls 109, 113 of each semiconductor fins 105 and 107. In one embodiment, the substrate 101 is dipped in a solution comprising a HF solution, which is diluted at a rate of 350:1. In another embodiment, the wet solution comprises any suitable solution being familiar to the skilled persons in the art. In some embodiments, the process step 207 comprises dry etching process being familiar to the skilled persons in the art.

Referring to FIG. 5, wafer 10 is placed into apparatus 300 as shown in FIG. 9, which is used for performing a plasma assisted deposition process. Apparatus 300 includes chamber 302, in which wafer 10 is placed, and power sources 304 and 306 connected into chamber 302. Wafer 10 comprises the semiconductor fins 105 and 107 on the substrate 101. Power source 304 may be a radio frequency (RF) power source with programmable pulse modulation function, while power source 306 may be a pulsed DC or RF power source for providing a bias voltage on the wafer 10. Power sources 304 and 306 may be operated independently from each other. Each of the power sources 304 and 306 may be programmed to be independently powered on and off without affecting the other.

Referring to FIG. 5 and FIG. 11, in process step 209, dopant-rich layer 131 is deposited on the top surface 115 and the sidewalls 113 of the second semiconductor fin 107 using production tool 300 as illustrated in FIG. 9. The dopant-rich layer 131 comprises a dopant that is used for forming the desirable LDD regions in the second semiconductor fin 107. Depending on the desirable conductivity type of the resulting FinFET, dopant-rich layer 131 may be formed of an n-type dopant (impurities) or a p-type dopant (impurities). For example, if the resulting FinFET is a p-type FinFET, dopant-rich layer 131 may comprise boron and/or indium, while if the resulting FinFET is an n-type FinFET, dopant-rich layer 131 may comprise phosphorous and/or arsenic. In an exemplary embodiment, the atomic percentage of the desirable dopant in dopant-rich layer 131 may be greater than about 90 percent, and may actually be a pure dopant layer.

The process gases in chamber 302 (refer to FIG. 9) may include AsH₃, B₂H₆, PH₃, BF₃, dilution gas such as Xe, Ar, He, Ne, H2 and/or the like, depending on the desirable composition of dopant-rich layer 131. The process may be performed under a pressure less than about 100 mTorr. RF power source 304 (FIG. 9) is turned on to generate plasma 308. The power of RF power source 304 may be between about 50 watts and about 1,000 watts, for example, although a greater or a smaller power may also be used. In an embodiment, RF power source 304 is turned on continuously during the entire period for forming dopant-rich layer 131. In an alternative embodiment, RF power source 304 is pulsed (in an on and off pattern) in order to improve the conformity (the step coverage) of dopant-rich layer 131.

During the formation of dopant-rich layer 131, DC power source 306 as in FIG. 9 has a low bias voltage lower than about 2 kV so that there is no unwanted amorphization layer formation during dopant layer formation. In an exemplary embodiment, the bias voltage output of DC power source 306 is between about 0 kV and about 2 kV. With the low or even zero DC bias voltage, the directionality of the ion doping process is reduced, and hence dopant-rich layer 131 may be deposited over the second semiconductor fin 107 as a separate layer, rather being directly implanted into fin 107. The DC bias voltage provided by DC power source 306 during the formation of dopant-rich layer 131 may also be pulsed (turned on and off) with a frequency of about 0.5 to about 10 KHz, as schematically illustrated in FIG. 10.

Referring to FIG. 6 and FIG. 11, in process step 211, a cap layer 133 is then deposited over the dopant-rich layer 131 on the second semiconductor fin 107. In some embodiments, the cap layer 133 may include silicon oxide, silicon nitride, silicon carbide or combinations thereof. The cap layer 133 has a thickness between about 30 Å to about 300 Å. Depending on cap layer 133 film density and compactness, the thickness of cap layer 133 should be controlled within a proper range. For example, in some embodiments, when the thickness is thinner than about 30 Å, the dopant in the dopant-rich layer 131 will diffuse out through the cap layer 133 during the following annealing process; when the thickness is thicker than about 300 Å, the cap layer 133 may peel off from the dopant-rich layer 131.

In one embodiment, the cap layer 133 is formed using plasma enhanced atomic layer deposition. Process precursor may include silanediamine, N,N,N′,N′-tetraethyl (sold as SAM 24 by Air Liquide) and O2. An operation power of the deposition is about 20 W to about 500 W. The cap layer 133 is formed at a temperature below about 300° C. to prevent the dopant in the dopant-rich layer 131 from diffusing out through the cap layer 133 during the cap layer formation process. In some alternative embodiments, the cap layer 133 may be deposited using other deposition techniques capable of forming a conformal layer of SiO₂, Si₃N₄, or SiC.

Referring to FIG. 7 and FIG. 11, in process step 212, the substrate 101 is annealed to form a LDD region 137 in the second semiconductor fin 107. The impurities in the dopant-rich layer 131 are activated and diffuse into the second semiconductor fin 107. The cap layer 133 prevents the dopant in the dopant-rich layer 131 from diffuse out through the cap layer 133 during the annealing process. The annealing may be performed at a wafer temperature between about 900° C. and about 1100° C. The annealing may be a millisecond annealing (MSA) or a rapid thermal annealing (RTA).

Next, in FIG. 11 of process step 213, the hard mask layer 129 in the first device region 100 is removed. Referring to FIG. 11, in process step 215, the cap layer 133 and the dopant-rich layer 131 are removed. In one embodiment, the process step 215 comprises dipping the substrate 101 in a wet solution comprising HF. In some embodiments, the process step 215 comprises etching the cap layer 133 and the dopant-rich layer 131 by a dry etching process.

FIG. 8 shows a substrate comprising a first fin 105 and a second fin 107 have LDD region 135 and LDD region 137 respectively. The LDD region 135 may be formed in the first semiconductor fin 105 using essentially the same process as discussed, except the second semiconductor fin 107 is covered by a hard mask during the formation of LDD region 137, and the first semiconductor fin 105 may have an opposite conductivity type than the second semiconductor fin 107. The dopant concentration in LDD regions may be between about 1E20/cm³ and about 1E21E/cm³, for example.

After the formation of LDD regions 135 and 137, gate spacers (not shown) may be formed. In subsequent process steps, n-type impurities (such as phosphorous) and p-type impurities (such as boron) may be implanted into fins 105 and 107, depending on the desirable conductivity types, to form deep source and drain regions (not shown). The dopant concentration in deep source/drain regions (not shown) may be between about 1×10²⁰/cm³ and about 1×10²¹/cm³, for example. FinFETs in the first device region 100 and the second device region 200 are thus formed.

FIGS. 1-11 are simplified for a better understanding of the inventive concepts of the present disclosure. For example, although the figures illustrate the FINFETs, it is understood the ICs may also include a number of various devices including resistors, capacitors, inductors, fuses, etc.

Various embodiments of the present invention may be used to moderate the shortcomings of the conventional LDD process. For example, in the various embodiments the formation of dopant-rich layer 131, the formation of cap layer 133 and the annealing process drive the impurities of the LDD region to a desirable depth without the concern of the shadowing effect and PAI induced twin boundary defects. Therefore, electrical performances of the devices can be improved.

One aspect of this description relates to a method of doping a fin field-effect transistor (FinFET). The method includes forming a plurality of semiconductor fins on a substrate wherein each semiconductor fin of the plurality of semiconductor fins has a top surface and sidewalls. The method further includes forming a gate stack over the top surface and sidewalls of each semiconductor fin of the plurality of semiconductor fins. The method further includes removing a portion a first semiconductor fin of the plurality of semiconductor fins exposed by the gate stack. The method further includes growing a first stressor region connected to a remaining portion of the first semiconductor fin. The method further includes exposing a second semiconductor fin of the plurality of semiconductor fins to a deposition process to form a dopant-rich layer comprising an n-type or a p-type dopant on the top surface and the sidewalls of the second semiconductor fin. The method further includes diffusing the dopant from the dopant-rich layer into the second semiconductor fin by performing an annealing process, wherein the first semiconductor fin is free of diffusion of the diffused dopant.

Another aspect of this description relates to a method of doping a fin field-effect transistor (FinFET). The method includes forming a plurality of semiconductor fins on a substrate wherein each semiconductor fin of the plurality of semiconductor fins has a top surface and sidewalls. The method further includes forming a gate stack over the top surface and sidewalls of each semiconductor fin of the plurality of semiconductor fins. The method further includes removing a portion of each semiconductor fin of the plurality of semiconductor fins exposed by the gate stack. The method further includes growing a first stressor region connected to a remaining portion of a first semiconductor fin of the plurality of semiconductor fins. The method further includes growing a second stressor region connected to a remaining portion of a second semiconductor fin of the plurality of semiconductor fins. The method further includes depositing a dopant-rich layer comprising an n-type or a p-type dopant on the top surface and the sidewalls of the second stressor region. The method further includes diffusing the dopant from the dopant-rich layer into the second semiconductor fin using an annealing process, wherein the first semiconductor fin is free of diffusion of the diffused dopant.

Still another aspect of this description relates to a fin field-effect transistor (FinFET). The FinFET includes a plurality of semiconductor fins on a substrate wherein each semiconductor fin of the plurality of semiconductor fins has a top surface and sidewalls. The FinFET further includes a gate stack over the top surface and sidewalls of each semiconductor fin of the plurality of semiconductor fins. The FinFET further includes a first stressor region connected to a portion of a first semiconductor fin of the plurality of semiconductor fins covered by the gate stack, wherein the first stressor region comprises a different material from the portion of the first semiconductor fin covered by the gate stack. The FinFET further includes a lightly doped drain (LDD) region in a second semiconductor fin of the plurality of semiconductor fins.

Although the embodiments and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, and composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps. 

What is claimed:
 1. A method of doping a fin field-effect transistor (FinFET), the method comprising: forming a plurality of semiconductor fins on a substrate wherein each semiconductor fin of the plurality of semiconductor fins has a top surface and sidewalls; forming a gate stack over the top surface and sidewalls of each semiconductor fin of the plurality of semiconductor fins; removing a portion of a first semiconductor fin of the plurality of semiconductor fins exposed by the gate stack; growing a first stressor region connected to a remaining portion of the first semiconductor fin; exposing a second semiconductor fin of the plurality of semiconductor fins to a deposition process to form a dopant-rich layer comprising an n-type or a p-type dopant on the top surface and the sidewalls of the second semiconductor fin; and diffusing the dopant from the dopant-rich layer into the second semiconductor fin by performing an annealing process, wherein the first semiconductor fin is free of diffusion of the diffused dopant.
 2. The method of claim 1, wherein growing the first stressor region comprises growing the first stressor region comprising silicon carbon (SiC).
 3. The method of claim 1, further comprising removing a portion of the second semiconductor fin of the plurality of semiconductor fins exposed by the gate stack.
 4. The method of claim 3, further comprising growing a second stressor region connected to a remaining portion of the second semiconductor fin of the plurality of semiconductor fins.
 5. The method of claim 4, wherein growing the second stressor region comprises growing the second stressor region comprising silicon germanium (SiGe).
 6. The method of claim 4, wherein diffusing the dopant from the dopant-rich layer into the second semiconductor fin comprises diffusing the dopant from the dopant-rich layer into the second stressor region.
 7. The method of claim 1, further comprising: covering the first stressor region with a second hard mask; exposing the first stressor region and the second hard mask to a deposition process to form a second dopant-rich layer comprising an n-type or a p-type dopant on the top surface and the sidewalls of the first stressor region; and diffusing the dopant from the second dopant-rich layer into the first stressor region by performing an annealing process.
 8. A method of doping a fin field-effect transistor (FinFET), the method comprising: forming a plurality of semiconductor fins on a substrate wherein each semiconductor fin of the plurality of semiconductor fins has a top surface and sidewalls; forming a gate stack over the top surface and sidewalls of each semiconductor fin of the plurality of semiconductor fins; removing a portion of each semiconductor fin of the plurality of semiconductor fins exposed by the gate stack; growing a first stressor region connected to a remaining portion of a first semiconductor fin of the plurality of semiconductor fins; growing a second stressor region connected to a remaining portion of a second semiconductor fin of the plurality of semiconductor fins; depositing a dopant-rich layer comprising an n-type or a p-type dopant on the top surface and the sidewalls of the second stressor region; and diffusing the dopant from the dopant-rich layer into the second semiconductor fin using an annealing process, wherein the first semiconductor fin is free of diffusion of the diffused dopant.
 9. The method of claim 8, wherein growing the second stressor region comprises growing the second stressor region comprising a different material from the first stressor region.
 10. The method of claim 8, further comprising protecting the first semiconductor fin using a hard mask during deposition of the dopant-rich layer.
 11. The method of claim 8, further comprising depositing a cap layer over the dopant-rich layer to prevent outward diffusion of the dopant.
 12. The method of claim 11, wherein depositing the cap layer comprises depositing the cap layer using plasma enhanced atomic layer deposition.
 13. The method of claim 8, wherein growing the first stressor region comprises growing the first stressor region comprising silicon carbon (SiC).
 14. The method of claim 8, wherein growing the second stressor region comprises growing the second stressor region comprising silicon germanium (SiGe).
 15. The method of claim 8, wherein diffusing the dopant from the dopant-rich layer into the second semiconductor fin comprises diffusing the dopant from the dopant-rich layer into the second stressor region.
 16. The method of claim 8, further comprising: depositing a second dopant-rich layer on the top surface and the sidewalls of the first stressor region, wherein the second dopant-rich layer comprises an n-type or a p-type dopant; and diffusing the dopant from the second dopant-rich layer into the first stressor region using an annealing process.
 17. A fin field-effect transistor (FinFET) comprising: a plurality of semiconductor fins on a substrate wherein each semiconductor fin of the plurality of semiconductor fins has a top surface and sidewalls; a gate stack over the top surface and sidewalls of each semiconductor fin of the plurality of semiconductor fins; a first stressor region connected to a portion of a first semiconductor fin of the plurality of semiconductor fins covered by the gate stack, wherein the first stressor region comprises a different material from the portion of the first semiconductor fin covered by the gate stack; and a lightly doped drain (LDD) region in a second semiconductor fin of the plurality of semiconductor fins.
 18. The FinFET of claim 17, further comprising a second stressor region connected to a portion of the second semiconductor fin covered by the gate stack, wherein the LDD region is in the second stressor region.
 19. The FinFET of claim 18, wherein the second stressor region comprises silicon germanium (SiGe).
 20. The FinFET of claim 17, wherein the first stressor region comprises silicon carbon (SiC). 